1. Field of the Invention
The present invention relates to a visual inspection method and visual inspection apparatus for inspecting an abnormality from the visual outside appearance of a target to be inspected. Here, the target to be inspected refers to, e.g., a semiconductor wafer, a photo mask, or a printed board. Also, the abnormality refers to, e.g., deformation, pattern defect, or foreign-substance mixing or adherence in fabrication processing steps of the to-be-inspected target.
2. Description of the Related Art
Conventionally, as the visual inspection method for inspecting a to-be-inspected target, there has been known the following method, for example: Namely, at first, image data is acquired while performing scanning on a semiconductor wafer on which one and the same pattern is repeatedly formed on plural chips. Next, a comparison is made between images on two chips which are adjacent to each other, thereby inspecting a defect from the visual outside appearance.
Moreover, as a method for making the comparison between the images, as is disclosed in, e.g., JP-A-61-212708, there has been already known the following method or the like: at first, after performing position alignment of the two pieces of image data, the difference therebetween is assumed. Next, difference-image data acquired is subjected to a threshold-value processing, thereby detecting the defect.
In the above-described image comparison method, detecting the abnormality on the visual outside appearance necessitates the following condition: The difference between images of an area where the abnormality exists, i.e., signal level of the defect, is larger enough as compared with the difference between images of an area where no abnormality exists, i.e., noise level of the images. In particular, as the abnormality gets smaller, the difference which occurs on the images becomes smaller because of a restriction on the resolution of an image-forming system. This results in a decrease in the detection sensitivity. On account of this situation, enhancing the detection sensitivity requires that the resolution of the image-forming system be enhanced, or that the noise level of the images be reduced.
The resolution of the image-forming system, however, is determined by wavelength of light used and performance of lenses in the case of, e.g., an image-forming optical system. Accordingly, there exists a limitation on expectation of the possible improvement. Also, the noise level of the images is determined by an image grabbing device, vibration of a stage where the to-be-inspected target is securely located at the time of grabbing the images, and further, the position alignment at the time of making the image-data comparison, and type of the to-be-inspected target. Consequently, there exists a limitation on the reduction in the noise level.